Among fluid flow sensors which are used in an air flow meter for measuring an intake air amount provided to an electronically-controlled fuel injection device for an internal-combustion engine in, e.g., vehicles, thermal fluid flow sensors are in the main stream because they can directly detect a mass amount of air.
Among those thermal fluid flow sensors, a thermal air flow sensor manufactured by the semiconductor micromachining technology particularly has attracted attention because it can reduce the manufacturing cost and can be driven with low power. As to such an air flow sensor, Japanese Patent Application Laid-Open Publication No. H11-194043 (Patent Document 1) discloses a technique of a diaphragm structure using platinum (Pt) for a heating element (heater) and a temperature detecting element (sensor), and eliminating a silicon (Si) film of the heater and a bottom portion of the sensor. In the diaphragm structure, layers above and below the heater and sensor are covered with an insulating layer and stress of these insulating layers combined is taken as a mild degree of tensile stress. Also, Japanese Patent Application Laid-Open Publication No. H11-271123 (Patent Document 2) discloses a configuration in which layers above and below a heater and a sensor are a compressive stress film each and a tensile stress film and these films are stacked to have top-bottom symmetry centering the heater in the configuration.
Japanese Patent Application Laid-Open Publication No. H11-295127 (Patent Document 3) relating to a flow sensor discloses a structure in which an insulating supporting film is formed on a surface of a plate-like base material, a plurality of heat sensitive resistive members are arranged on the supporting film, and the heat sensitive resistive members and the supporting film are covered with an insulating protective film.
Japanese Patent Application Laid-Open Publication No. 2002-131106 (Patent Document 4) relating to a thermal air flow meter discloses a structure of a micro heater having a thin-film heating portion provided on an upper surface of a cavity that is formed in a single crystal silicon substrate, the micro heater heating flowing gas to be measured. In the micro heater, the thin-film heating portion is configured by a heating resistive member, a temperature-measuring resistive member, an air-temperature-measuring resistive member, and a top thin film and a bottom thin film which are provided to sandwich these resistive members from above and below. Also, at least one of the top thin film and the bottom thin film is formed to include a tensile stress film and a water-resistance compressive stress film is formed to be stacked on a side of the tensile stress film facing the flowing gas.